Most of this code requires using the Genesis Framework since that is mostly what I work with. So assume that any PHP should go into the child theme's functions file, functions.php (or page template where specified), and style rules go in the style sheet, usually style.css.

$prev=$wpdb->get_row($wpdb->prepare("SELECT ID, post_title FROM $wpdb->posts WHERE post_type='post' AND post_status='publish' AND post_author='".$post->post_author."' AND post_date < '".$post->post_date."' ORDER BY post_date DESC LIMIT 1;"));

$next=$wpdb->get_row($wpdb->prepare("SELECT ID, post_title FROM $wpdb->posts WHERE post_type='post' AND post_status='publish' AND post_author='".$post->post_author."' AND post_date > '".$post->post_date."' ORDER BY post_date ASC LIMIT 1;"));